Ships comprising combustion engines that run on natural gas and propel these ships are known. In order to ensure correct operation of such combustion engines, the combustible natural gas needs to have certain properties. In particular, the methane number needs to be above a threshold, generally specified by the engine manufacturer, because gases with excessively low methane numbers do not burn correctly and cause engine knocking.
In order to guarantee that the natural gas fed to the engine has a methane number above a threshold, it is known for the engine supply circuit to be fitted with a phase separator that allows the heavy hydrocarbons of the natural gas to be returned to the storage tank in the form of condensate.
However, returning heavy hydrocarbons causes them to build up in the tank. Thus, as the level in the tank gets close to the bottom, a residue of heavy hydrocarbons remains that cannot be used for ship propulsion.